A compression rod is a type of mechanical linkage that is configured to deform upon application of a predetermined force. Compression rods are often used in such applications as fixed wing and rotary wing aircrafts, wherein compression rods are used to connect control devices that enable the aircraft to be maneuvered. Buckling is a phenomenon in compression rods whereby a mechanical structure deforms under axial, compressive loading, but because of the deformation there is a critical load beyond which the rod cannot carry additional load. If friction is introduced by bearings positioned at each end of the rod, the rod can hold a higher load than the predicted critical buckling load until the static frictional force is overcome. When the static frictional force is overcome, buckling commences, and the load immediately drops to the predicted critical buckling load, wherein large deformations are evident.
When a predetermined, axially oriented force is applied to a compression rod, buckling generally needs to be avoided in order for the rod to carry the load safely. Such avoidance is accomplished by increasing the area of the rod's cross-section, thereby increasing the weight of the rod. There are times, however, when, to prevent damage to more expensive components that the compression rod is coupled to the less expensive compression rod is sacrificed. In this case, the compression rod must safely carry the required loads, but if loads become higher than a maximum limit, the rod buckles to protect the attached component from experiencing loads beyond a threshold, as such loads would cause irreparable damage.
The critical load that causes buckling is calculated for a compression member for the case of frictionless ends (i.e. cannot support moments). The critical load that causes buckling, Fcrit, is given by the equation
            F      crit        =                  π        ⁢                                  ⁢        EI                              (          KL          )                2              ,wherein E is equal to the modulus of elasticity of the compression rod, I is equal to the second moment of inertia of the compression rod, L is equal to the length between the centers of the bearings, K is equal to 1.0 for hinged ends wherein friction does not exist and to 0.5 for situations wherein both ends are fixed (i.e., bearings are locked), r is equal to the radius of gyration and is defined by
      I    A  wherein A is equal to cross sectional area. The slenderness ratio is defined by L/r. Compression rods having a slenderness ratio above 200 are said to be long and/or slender. Compression rods having a slenderness ratio between 50 and 200 are said to be an intermediate length.
When bearings are positioned on ends of the compression rod, the critical load that causes buckling is difficult to predict. Many factors, including the eccentricity of the point of load application to the center of the rod's cross-section, influence the load at which the compression rod will fail. Even between two similar compression rods with similar frictional bearings on each end, the load at which each fails may be drastically different from the predicted failure load. In addition, similar compression rods with frictional bearings tend to lack repeatable actual failure loads. Those skilled in the relevant art have long sought, but have been unable to arrive at, a compression rod that consistently fails within a narrow range of the predicted failure load for the compression rod.